There has recently been a rapid increase in demand for .alpha.-L-aspartyl-L-phenylalanine methyl ester (Aspartame), an artificial dipeptide sweetening agent. N-formyl-.alpha.-aspartyl phenylalanine is an important compound as an intermediate for the preparation of Aspartame. Specifically, Aspartame can be prepared by de-formylating N-formyl-.alpha.-aspartyl phenylalanine in a methanol-hydrochloric acid solution, followed by esterification (Japanese Patent Publication No. 26133/1980 and Japanese Patent Laid-Open No. 82752/1978).
Only Japanese Patent Publication No. 26133/1980 discloses a process for preparing N-formyl-.alpha.-aspartyl phenylalanine by condensating N-formyl aspartic acid anhydride and phenylalanine. This process comprises condensating N-formyl aspartic acid anhydride with phenylalanine in glacial acetic acid and, more specifically, comprises effecting the condensation reaction at 45.degree.-50.degree. C. by adding L-phenylalanine little by little to a mixture having N-formyl aspartic acid anhydride suspended in glacial acetic acid present in an amount ten times as great as the N-formyl aspartic acid anhydride.
In the condensation of N-formyl aspartic acid anhydride and phenylalanine, N-formyl-.beta.-aspartyl phenylalanine (.beta.-isomer), a product of the condensation of phenylalanine with .beta.-carboxylic acid of aspartic acid, is simultaneously formed in addition to the intended N-formyl-.alpha.-aspartyl phenylalanine (.alpha.-isomer). However, only the ester derived from the .alpha.-isomer is useful as a sweetening agent while the ester derived from the .beta.-isomer is an unfavorable substance since it has no sweetening effect but on the contrary exhibits a bitter taste.
In accordance with the disclosure of the above-described prior art, the condensation reaction of N-formyl aspartic acid anhydride and phenylalanine by-produces the .beta.-isomer in such amounts of 20% or more. Accordingly, in order to produce the sweetening agent Aspartame at low cost on an industrial scale, it is necessary to re-utilize the .beta.-isomer by separating it from the 60 -isomer and hydrolyzing it into aspartic acid and phenylalanine for reuse as a raw material.
However, in the process disclosed in Japanese Patent Publication No. 26133/1980 wherein the condensation reaction is practiced in acetic acid, it is necessary to treat the solvent acetic acid, for example, by removing it from the filtrate containing primarily the .beta.-isomer, which has been separated with the .alpha.-isomer after the reaction, prior to the recovery of aspartic acid and phenylalanine from the .beta.-isomer by hydrolysis. In other words, the prior art process has the disadvantages of involving complicated procedures.